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LWN on Security: Updates, fs-verity, Spectre, Qubes OS/CopperheadOS

Submitted by Roy Schestowitz on Thursday 13th of September 2018 03:05:13 AM Filed under
Linux
Security
  • Security updates for Wednesday
  • Protecting files with fs-verity

    The developers of the Android system have, among their many goals, the wish to better protect Android devices against persistent compromise. It is bad if a device is taken over by an attacker; it's worse if it remains compromised even after a reboot. Numerous mechanisms for ensuring the integrity of installed system files have been proposed and implemented over the years. But it seems there is always room for one more; to fill that space, the fs-verity mechanism is being proposed as a way to protect individual files from malicious modification.

    The core idea behind fs-verity is the generation of a Merkle tree containing hashes of the blocks of a file to be protected. Whenever a page of that file is read from storage, the kernel ensures that the hash of the page in question matches the hash in the tree. Checking hashes this way has a number of advantages. Opening a file is fast, since the entire contents of the file need not be hashed at open time. If only a small portion of the file is read, the kernel never has to bother reading and checking the rest. It is also possible to catch modifications made to the file after it has been opened, which will not be caught if the hash is checked at open time.

  • Strengthening user-space Spectre v2 protection

    The Spectre variant 2 vulnerability allows the speculative execution of incorrect (in an attacker-controllable way) indirect branch predictions, resulting in the ability to exfiltrate information via side channels. The kernel has been reasonably well protected against this variant since shortly after its disclosure in January. It is, however, possible for user-space processes to use Spectre v2 to attack each other; thus far, the mainline kernel has offered relatively little protection against such attacks. A recent proposal from Jiri Kosina may change that situation, but there are still some disagreements around the details.

    On relatively recent processors (or those with suitably patched microcode), the "indirect branch prediction barrier" (IBPB) operation can be used to flush the branch-prediction buffer, removing any poisoning that an attacker might have put there. Doing an IBPB whenever the kernel switches execution from one process to another would defeat most Spectre v2 attacks, but IBPB is seen as being expensive, so this does not happen. Instead, the kernel looks to see whether the incoming process has marked itself as being non-dumpable, which is typically only done by specialized processes that want to prevent secrets from showing up in core dumps. In such cases, the process is deemed to be worth protecting and the IBPB is performed.

    Kosina notes that only a "negligible minority" of the code running on Linux systems marks itself as non-dumpable, so user space on Linux systems is essentially unprotected against Spectre v2. The solution he proposes is to use IBPB more often. In particular, the new code checks whether the outgoing process would be able to call ptrace() on the incoming process. If so, the new process can keep no secrets from the old one in any case, so there is no point in executing an IBPB operation. In cases where ptrace() would not succeed, though, the IBPB will happen.

  • Life behind the tinfoil curtain

    Security and convenience rarely go hand-in-hand, but if your job (or life) requires extraordinary care against potentially targeted attacks, the security side of that tradeoff may win out. If so, running a system like Qubes OS on your desktop or CopperheadOS on your phone might make sense, which is just what Konstantin Ryabitsev, Linux Foundation (LF) director of IT security, has done. He reported on the experience in a talk [YouTube video] entitled "Life Behind the Tinfoil Curtain" at the 2018 Linux Security Summit North America.

    He described himself as a "professional Russian hacker" from before it became popular, he said with a chuckle. He started running Linux on the desktop in 1998 (perhaps on Corel Linux, which he does not think particularly highly of) and has been a member of the LF staff since 2011. He has been running Qubes OS on his main workstation since August 2016 and CopperheadOS since September 2017. He stopped running CopperheadOS in June 2018 due to the upheaval at the company, but he hopes to go back to it at some point—"maybe".

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